And now after a mammoth nine-and-a-half-year mission in search of potentially life-sustaining planets, NASA's Kepler Space Telescope has officially run out of fuel.
Kepler is at the verge and waiting for the command which would come in the next two weeks that will deactivate its transmitter and instruments after which it will drift in a safe orbit trailing the Earth.
"As NASA's first planet-hunting mission, Kepler has wildly exceeded all our expectations and paved the way for our exploration and search for life in the solar system and beyond", Thomas Zurbuchen, associate administrator of NASA's Science Mission Directorate, said in a statement. Those systems range from Kepler-233, whose parent star may be merely 5 million to 10 million years old, to Kepler-444, whose planets may be more than twice Earth's 4.5 billion-year age.
Kepler showed us that "we live in a galaxy that's teeming with planets, and we're ready to take the next step to explore those planets", she said.
Kepler allowed astronomers to discover that 20% to 50% of the stars we can see in the night sky are likely to have small, rocky, Earth-size planets within their habitable zones - which means that liquid water could pool on the surface, and life as we know it could exist on these planets.
Four years into its mission, mechanical failures briefly halted observations. "It was an extremely clever approach to doing this kind of science", said Leslie Livesay, director for astronomy and physics at NASA's Jet Propulsion Laboratory, who served as Kepler project manager during mission development. Over the life of the mission, more than 100,000 of those stars were actively monitored by Kepler.
Kepler focused on stars thousands of light-years away and, according to NASA, showed that statistically there's at least one planet around every star in our Milky Way Galaxy.
Lion Air plane had airspeed problem on flight prior to crash
The crew received the green light to turn around, but lost altitude and crashed into the sea less than 15 minutes into the flight. The flight data recorder contains technical information about the flight, detailed by the plane's onboard flight systems.
The asteroid-hopping Dawn mission ran out of fuel last month. Scientists are expected to spend over a decade making new discoveries in the treasure trove of data Kepler provided.
The latest data, from Campaign 19, will complement the data from NASA's newest planet hunter, the Transiting Exoplanet Survey Satellite, launched in April. "Kepler has had a successful nine-and-a-half year mission". The first data from TESS is already being sent to Earth and analyzed.
There's poetry in Kepler's ability to make us feel both small and also so connected to the rest of our universe.
Kepler's data also provided a new way to assess whether a planet had a solid surface, like Earth and Mars, or is gaseous, like Jupiter and Saturn. Rather, it served as a sensitive photometer, continuously measuring the brightness of stars in its wide field of view, on the lookout for the tell-tale dimming that occurs when a planet passes in front of its sun as viewed from Earth. They convinced their peers and they convinced NASA that this was a mission that had to be done.
"In the end, we didn't have a drop of fuel left for anything else", said Charlie Sobeck, project system engineer.
Kepler's originally planned $640 million mission went far longer than anticipated, thanks in part to a spacecraft-saving fix that was made in 2013 when a crucial part of the probe's fine-pointing system went out of commission.
NASA's Kepler mission has discovered a world where two suns set over the horizon instead of just one.